The present invention relates to a lighting system utilizing the sunlight, which may collect the sunlight at a place which is normally showered with the sun rays and transmit the collected sunlight to another place where the direct access to the sun rays is impossible.
Solar energy has been considered and is still considered to become available as the most promising and important source of infinite and pollution-free energy by the end of the 20th century for all the people living on the Earth. However, the solar constant is so low than an extremely large solar energy collection installation is required so that the collected solar energy may be converted into electrical energy in sufficient quantity so that it may be used successfully in practice. For instance, a large number of solar cells are arrayed in a vast area so as to convert the solar energy into electrical energy which in turn is transmitted to a desired destination.
Another serious problem encountered in the practical utilization of solar energy is a low conversion efficiency with which the solar energy is converted into electric energy. Furthermore, the electric energy thus obtained is further converted into other useful forms of energy such as mechanical energy, light energy and so on. Thus low conversion efficiencies are accumulated resulting in the use of only an extremely small fraction of the solar energy collected.
Meanwhile, large buildings have been constructed in urban areas and these buildings are in general built airtight in order to economize the energy required for air conditioning. As a result there has been a problem of how to provide highly reliable and dependable emergency stand-by lighting or illumination systems at emergency exits along emergency staircases and in totally enclosed rooms so as to ensure safe and quick evacuation of people inside buildings in the case of an emergency such as a fire, an earthquake or the like. Furthermore, there has been an increasing demand for inexpensive, safe and simple lighting or illumination systems for tunnels and mine pits.
In the conventional emergency stand-by lighting or illumination systems, there is provided a stand-by electric source such as batteries or generators independently of the commercial electric supply. In addition, emergency lamps must be provided independently of the lighting or illumination systems normally used. Thus the inspection and maintenance of the emergency stand-by lighting or illumination systems are very cumbersome and expensive. Furthermore, there is a fear that the stand-by electric sources and illumination lamps may be damaged in the case of an earthquake, and the service life of the electric source is in general short. Thus the conventional emergency stand-by lighting or illumination systems are not completely reliable. Especially in the case of the emergency stand-by lighting or illumination systems installed in large buildings, the reliability thereof is of great importance during the daytime when concentrations of people are extremely high. Thus the conventional emergency stand-by lighting or illumination systems have many problems to be overcome.
In the case of the lighting or illumination systems for tunnels and pits, electric lighting or illumination has been widely used so that the high cost of electricity presents a problem. When a private electric installation is used because for instance a commercial electric supply is not readily available or for other purposes, the installation cost as well as the maintenance cost becomes very high.
Meanwhile for use in optical communications systems which are much more efficient and economical, extensive research and development respecting optical fibers has been conducted. Now optical fibers are readily available which are highly transparent, with resultant reduction in transmission or propagation losses. In practice, bundles of optical fibers have been used for directing illuminating light from one place to another. Thus with the optical fibers or optical conductors the solar light energy collected may be transmitted from one place to another at a high efficiency. However, the optical fibers for optical communications use available in the market are in general about 0.1 mm in diameter. It is apparent that it is extremely difficult to transmit the highly concentrated solar light energy through an optical fiber of such extremely small diameter. When light from the Sun is focused through a conventional optical system, its image has a diameter far greater than about 0.1 mm so that the solar energy cannot be introduced into the optical fiber. Thus with conventional optical fibers, the effective transmission of the collected solar energy from one place to another is almost impossible in practice.